Defibrillation electrode switch condition sensing

ABSTRACT

Method and apparatus for detecting the open or closed state of a pair of manually-operated switches 116, 118 associated with a pair of defibrillation pulse electrodes, the switches being normally open and being manually closed by an operator, the two switches connected in series with discharge control circuitry 32 for initiating a defibrillation energy pulse. The control circuitry 32 is configured so that when the switches 116, 118 are both closed a defibrillation pulse is initiated. Each of the switches is connected in parallel with a corresponding circuit element 124, 126, each having a characteristic impedance, and being shorted out when the corresponding switch 116, 118 is closed. The invention provides for measuring an indication of the series impedance of the discharge control circuitry 32 to determine the open or closed state of the switches by comparing a measured impedance value indication with an impedance value indication calculated for a given open or closed state of the switches.

This invention relates to transcutaneous pacemakers and defibrillators.

Emergency techniques for cardiac therapy are essential for successfullytreating life threatening cardiac conditions. The most common of suchconditions is ventricular fibrillation, in which the electrical pulsegenerators in the cardiac muscle fibrillate asynchronously, causingchaotic muscle contraction. The other common cardiac threat is loss ofpacing, in which the pacing stimulus nerves of the cardiac muscle failto initiate contraction of the muscle.

Ventricular fibrillation is treated with a high energy electrical pulse,called a defibrillation pulse, which is transcutaneously delivered tothe heart to resynchronize the heart's pulse generators. Loss of pacingis treated using a transcutaneous pace maker to deliver pacing currentpulses to the heart and thereby maintain cardiac contractions.

Frequently a patient in cardiac distress experiences both the conditionsof fibrillation and loss of pacing. In order to most efficiently treatthis situation, both defibrillation and external pacing equipment aretypically combined in a single portable instrument for emergencypersonnel convenience. Such an instrument includes a pair oftranscutaneous pacing electrodes and corresponding pacing circuitry, aswell as a pair of defibrillation electrodes and correspondingdefibrillation circuitry. In addition, the instrument may include aspecialized multifunction electrode pair which can deliver both pacingand defibrillation pulses when used with appropriate connections to thepacing and defibrillation circuitry. In either case, a hardware relayscheme typically isolates operation of the pacing circuitry from thedefibrillation circuitry.

Typically, a hardware relay scheme isolates operation of the pacingcircuitry from the defibrillation circuitry to thereby isolate thecircuitry outputs. When separate pairs of pacing and defibrillationelectrodes are used with the circuitry, the relay scheme decouples thetwo circuits. Conversely, when the multifunction electrode pair is used,the relay scheme couples the circuits to a common output so that thepacing stimuli and defibrillation pulses may be delivered to a singleelectrode pair. Such relays, being required to withstand thedefibrillation pulse, are quite bulky and rather expensive, and must beisolated from any surrounding transformers that could trigger theiractivation.

Defibrillation electrodes are usually mounted one each on a hand-heldpaddle which includes a pressure-activated switch for initiating thedefibrillation discharge of energy to a patient. Because this energy ishigh enough to be lethal if delivered at the wrong time or to the wronglocation, the paddle switches are both connected in series with thedischarge circuitry. If one switch is unintentionally activated alone,no discharge will result; only the simultaneous activation of bothswitches will initiate a discharge. However, if one switch is heldclosed (activated) as a result of a hardware malfunction, activation ofthe other switch would result in unintentional defibrillation discharge.

The electrical current generated by the defibrillation discharge circuitfor delivery to the patient is typically monitored to check theintegrity of the circuit and to study the physiological effects of thedischarge. This current may reach as much as 125 A, and thecorresponding voltage of the discharge may reach as much as 5000 V.Conventionally, the current is monitored using a current transformerwhose primary winding is connected in series with the discharge circuit.The secondary winding of the transformer is then connected to a sensingcircuit for measuring the current level. The particular choice oftransformer is based on the requirement that the component withstand thehigh current and voltage levels of discharge circuit.

It is desirable to frequently test the defibrillator discharge circuitfor functionality, due to the critical nature of the emergencysituations in which it is needed. Traditionally, such a test isaccommodated by providing a characteristic load, say 50 Ω, into whichthe circuit may be energized to simulate discharge into a patient. The50 Ω load resistor is typically contained either within thedefibrillator equipment or within a separate testing apparatus, with aconnection through the equipment housing for contact to thedefibrillator electrodes.

Because the defibrillator discharge's high energy (about 360 j) isdelivered in a short time (90% in about 3 msec), the peak powerrequirement of the load resistor exceeds 100 KW. The resistor must bewell ventilated because the average power may exceed 30 watts, resultingin significant resistive heating. Additionally, a sizeable resistor isrequired to withstand the discharge's peak voltage of as much as 2500 V.The high voltage also requires the electrical connection from thedefibrillator electrodes to the resistor to regularly withstand suchvoltage. Finally, the electrical connection on the equipment housing tothe resistor must be isolated to prevent accidental contact by anoperator; discharge of the circuit with only one electrode connected tothe resistor while an operator touches the resistor would dump the highvoltage across the resistor to the operator.

SUMMARY OF THE INVENTION

In general, in one aspect, the invention features a scheme for detectingthe open or closed state of a pair of manually-operated switchesassociated with a pair of defibrillation pulse electrodes, the switchesbeing normally open and being configured to be manually closed by anoperator, the two switches connected in series with discharge controlcircuitry for initiating a defibrillation energy pulse. In theinvention, the control circuitry is configured so that when the switchesare both closed a defibrillation energy pulse is initiated. Each of theswitches is connected in parallel with a corresponding one of first andsecond circuit elements, each having a characteristic impedance, so thateach switch shorts out a corresponding circuit element when that switchis closed. The invention provides for measuring an indication of theseries impedance of the discharge control circuitry to thereby determinethe open or closed state of the switches by comparing a measured seriesimpedance value indication with a series impedance value indicationcalculated for a given open or closed state of each of the switches.Further, the invention provides for detecting the closed state of eitherof the switches at a time when the switches should be open andindicating such a failure detection to the defibrillation controlcircuit user, and preventing a discharge of a defibrillation pulse ondetection of such a failure. The invention thus provides the user withthe capability of monitoring the integrity of the switches, and preventsagainst false discharges of the defibrillation pulse discharge circuit.

In preferred embodiments, the discharge control circuitry includes anelectrical component switch that is normally inactivated and that can beelectronically activated, the electrical switch being connected inseries with the discharge control circuitry so that when the twomanually-operable switches are simultaneously closed and the electricalswitch is activated the discharge control circuitry initiates a saidenergy pulse; a third circuit element is connected in parallel with theelectrical component switch and has a characteristic impedance, so thatthe component switch shorts out the third circuit element when thecomponent switch is activated; and the invention provides for detectingthe activated state of the electrical switch at a time when saidelectrical switch should be inactivated and for indicating such afailure detection to the circuit user, and preventing a discharge of asaid defibrillation pulse on detection of such a failure.

In more preferred embodiments, the series impedance indication is basedon a measured voltage drop across the switches and each of the first,second, and third circuit elements comprises a resistor; the firstresistor and the second resistor each have the same resistance value,and the open or closed state of the series combination of the twomanually-operable switches is determined; and each electrode and itsassociated manually-operable switch is located on a hand held paddle.

Other features and advantages of the invention will be apparent from thefollowing description of a preferred embodiment and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a portable transcutaneous pace maker anddefibrillator unit.

FIG. 2 is a schematic of the pacing circuitry and defibrillationcircuitry in the unit of FIG. 1 and illustrates the circuitryinterconnection of the invention.

FIG. 3A is a perspective view of a conventional cable connector for usewith the unit of FIG. 1.

FIG. 3B is a perspective view of a multifunction cable connector for usewith the unit of FIG. 1.

FIG. 4A is a perspective view of an instrument connector for use withthe connectors of FIG. 3.

FIG. 4B is a cross sectional view taken at 4B--4B in FIG. 4A of two highvoltage connection pins used in the cable connector of FIG. 4A.

FIG. 5 is a schematic of the defibrillator electrode switches of theinvention.

FIG. 6 is a schematic of the defibrillation discharge circuit andsensing circuit of the invention.

FIG. 7A is a perspective view of the inductor in the circuit of FIG. 6.

FIG. 7B is a perspective view of the inductor of FIG. 7A and the sensewinding in the circuit of FIG. 6.

FIG. 8 is another perspective view of the pace maker and defibrillatorunit of FIG. 1, taken from another angle and showing, with portionspartially broken away, a short circuit bar for use with thedefibrillator paddles.

FIG. 9A is a plot of the voltage waveform developed by the sense windingof FIG. 7 in response to the current in the inductor of FIG. 7.

FIG. 9B is a plot of the current waveform corresponding to the voltagewaveform of FIG. 9A when that waveform is integrated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown a portable defibrillator and pacingunit 10 for providing defibrillation energy pulses and transcutaneouspacing stimuli to a patient in cardiac distress. Such a unit isavailable from Zoll Medical Corporation of Woburn, Mass. under theproduct name PD1400.

The portable unit 10 includes a cable connector 12 for connecting apacing electrode cable 14 and defibrillation electrode cables 16 topacing circuitry and defibrillation circuitry, respectively (shown inFIGS. 2 and 6), located within the unit. Hand held defibrillationelectrode paddles 18, which mechanically support the defibrillationelectrodes (shown in FIG. 8) are connected to the opposite end of thedefibrillation electrode cables.

A screen 20 is provided on the portable unit for displaying operationalmodes and test results, which are programmed by variousoperator-controlled switches and knobs 22. Finally, the portable unitincludes a power supply compartment 24 for holding a portable battery orother power source.

Referring to FIG. 2, the defibrillation circuitry 30 included in theportable unit comprises a discharge energy generator 32 for charging adischarge capacitor 34 to a defibrillation discharge energy specified bythe operator. A switch 36 connects the capacitor to the discharge energygenerator, and when the switch is activated by control circuitry (notshown) discharges the capacitor across the positive and negativedefibrillation outputs 40A and 40B, respectively. The capacitor ispreferably of 46 microfarads and can withstand 5000 volts across itsterminals. An inductor 38 is in series with the capacitor for shapingthe defibrillation pulse as the capacitor is discharged. The inductor ispreferably of 20 millihenries and has an associated winding resistanceof about 13 Ω.

The pacing circuitry 42 included in the portable unit comprises anelectronic pace generator 44 for generating pacing stimuli and timingthe delivery of the stimuli at the pacing circuitry positive andnegative pacing outputs 46A and 46B, respectively. In series with thepositive output 46A is a positive blocking diode 48A connected so thatit is forward biased by the positive sense of the pace stimuli.Similarly, in series with the negative output 46B is a negative blockingdiode 48B connected so that it is forward biased by the negative senseof the pace stimuli. The blocking diodes act to isolate the pacegenerator 44 from the typically high energy defibrillation dischargepulses, as discussed below. Preferably, both of the blocking diodes canwithstand a reverse voltage of at least 5000 V and a forward currentsurge of at least 25 A; such a surge could occur if the defibrillationcircuit is unintentionally reverse connected to the pacing circuit.

Both the positive and negative defibrillation circuitry outputs 40A and40B and the positive and negative pacing circuitry outputs 46A and 46Bare tapped into an instrument connector 49 in the portable unit housingfor connection with either a conventional mating connector 12, shown inFIG. 1, or a multifunction mating connector 53, described below. In theconventional mating connector 12 two pins 50A and 50B correspond to thepositive and negative defibrillation circuitry outputs 40A and 40B,respectively, while two pins 52A and 52B correspond to the positive andnegative pacing circuitry outputs 46A and 46B, respectively. Thedefibrillation pins 50A and 50B are connected to defibrillation cables16 and corresponding electrode paddles 18, as shown in FIG. 1, forapplication of defibrillation discharge pulses to a patient. Likewise,the pacing pins 52A and 52B are connected to a pacing cable 14 forconnection to pacing electrodes positioned on a patient.

In the multifunction mating connector 53, two pins 54A and 54Bcorrespond to the positive and negative defibrillation circuitry outputs40A and 40B, respectively, while two pins 56A and 56B correspond to thepositive and negative pacing circuitry outputs 46A and 46B,respectively, just as in the conventional mating connector. However, inthe multifunction connector the positive defibrillation output pin 54Aand the positive pacing output pin 56A are connected together;similarly, the negative defibrillation output pin 54B and negativepacing output pin 56B are connected together.

The positive pin connection 54A to 56A and the negative pin connection54B to 56B each are connected to a single positive cable line 58A and asingle negative cable line 58B, respectively; the single pair of cablelines carry either defibrillation discharge pulses or pacing stimuli toa pair of multifunction electrodes (not shown) via a multi use connector60. This multifunction electrode scheme provides an advantage over theconventional connector and electrode scheme by requiring only one pairof electrodes to be positioned on a patient, rather than both adefibrillation pair and a pacing pair. However, by connecting thepositive circuitry pins together in the multifunction connector 53itself, rather than in the instrument connector 49, the circuitryconnector 49 is compatible with both the conventional and themultifunction electrode schemes and the operator may choose one asappropriate.

When in the multifunction connector and electrode configuration, outputsignals of the defibrillation circuitry are coupled to the pacingcircuitry, and output signals of the pacing circuitry are coupled to thedefibrillation circuitry. Thus the connector and circuit wiring must beable to carry the 25 A potentially developed in the discharge circuitry.Furthermore, the positive and negative connection pins in the connectorsmust meet high voltage and current specifications, as discussed below.

High energy discharge pulses generated by the discharge circuitry woulddamage the pace generator 44 of the pace circuitry if it were notisolated by the blocking diodes 48A and 48B. While the diodes arepositioned to be forward biased by the pace generator pacing stimuli,the diodes are reversed biased with respect to the defibrillationcircuit discharge pulses, and so block these pulses from the pacegenerator. Conversely, pacing stimuli generated by the pacing circuitare not blocked from coupling to the discharge circuit because thedischarge circuit comprises only one passive component, an inductor,when the pacing circuit is operating. Because the inductor is designedto withstand the high energy defibrillation pulses, it easily withstandsthe much lower energy pacing stimuli. Thus, without the use ofmechanical isolators such as relays, the blocking diodes isolate thepacing circuitry from the high energy defibrillation circuitry, whileeliminating the typical space requirements and cost of mechanicalisolators.

Referring now to FIG. 3A, the conventional mating connector 12, alsoshown in FIG. 1, is connected to a pacing cable 14 which delivers thepacing circuitry outputs 46A and 46B (FIG. 2) to pacing electrodesplaced on a patient. Separate defibrillation cables 16 deliver thedefibrillation circuitry outputs 40A and 40B to defibrillation electrodepaddles for placement on the patient. The two sets of cables and thedisconnected output pins in the connector isolate the pacing anddefibrillation circuitry from each other.

As shown in FIG. 3B, the multifunction mating connector 53 is connectedto a single cable 58 for delivering both defibrillation pulses andpacing stimuli on a single positive cable line 58A and a negative cableline 58B (FIG. 2) The cable 58 leads to a multifunction electrodeconnector 60 which attaches to a single pair of patient electrodes forboth transcutaneous pacing and defibrillation. The blocking diodes inthe pacing circuit are the only means used for isolating the dischargepulses from the pacing circuitry in this multifunction scheme.

The instrument connector 49, shown in FIG. 4A with its cover 62 pulledback, includes a number of pin connections, four of which are highvoltage pins 63 positioned in high voltage labyrinth barrels. Each ofthe four barrels corresponds to one of the four outputs from the pacingand defibrillation circuitry, the positive and negative dischargeoutputs 40A and 40B and the positive and negative pacing outputs 46A and46B. The other pins correspond to low voltage control leads and otherwiring necessary for the circuitry.

When mated with either the conventional connector or the multifunctionconnector, the instrument connector high voltage barrel pins 63 matewith corresponding mating pins 66. The pin connections meet the IEC 601Part 2 safety specification for defibrillation equipment; two adjacentpin connections can withstand the application of 7500 V between them,and the shortest electrical path 68-70 between two adjacent pinconnections, called the creepage distance, is not less than 15millimeters.

Referring now to FIG. 5, a relay coil or solenoid 81 actuates themedical grade relay switch 36 (FIG. 2). The relay coil is connected atone of its terminals to a dual 10 V and 20 V power supply 82, 84, and a470 microfarad capacitor 90 connected to ground 92 for increasing thespeed of the relay switch activation. The relay coil is actuated whenits other terminal is connected to ground by the simultaneous closure ofthree switches. One of these switches is a MOS transistor switch 94whose gate 96 is connected to a control circuit such as amicroprocessor. A 56 KΩ shunt resistor 98 is connected across the MOStransistor for sensing the on or off state of the transistor, asdiscussed below. The MOS transistor is biased by a 2 KΩ resistor 100 anda 100 KΩ resistor 102.

The positive input of an operational amplifier 104 is connected to theMOS transistor sense network 100, 102 and a zener diode 106 having a 4.3V avalanche voltage. The negative input and the output of theoperational amplifier 104 are both connected to a network of two 2 KΩresistors 108, 110 and a 0.1 microfarad capacitor 112 which together actas a voltage filter. In this configuration, the operational amplifieracts as a voltage follower for sensing the voltage level of the junction97 of the MOS transistor and the defibrillation paddle switches, asdescribed below. The voltage sense line 114 is connected to the samemicroprocessor that is connected to the gate of the MOS transistor, forcontrol of the transistor, as described below. The microprocessordigitizes the analog sense voltage and activates the transistoraccordingly.

The MOS transistor 94 and its shunt resistor 98 are connected to a pinin the instrument connector 49 (FIG. 2). When this connector is matedwith either the conventional mating connector 12 (as shown) or themultifunction connector, the MOS transistor and its shunt resistor areconnected in series with two mechanical switches 116, 118. The switchesare rated to withstand the 1 A required by the discharge relay coil.Using the conventional connector 12, the first switch 116 is located inone electrode paddle 120 while the second switch 118 is located in theother electrode paddle 122. Conversely, if the multifunction connectoris used as shown in FIG. 3B, both switches 116, 118 are connected tobuttons 55 located on the multifunction connector housing 53. In eithercase, the switches are pressure activated by pressing a depressible pushbutton. The switches are electrically connected in series via a jumperwire located in the connector; both the conventional and multifunctionconnectors provide this jumper wire. One of the other switch terminalsis connected to ground via another pin in the connector and thecorresponding pin in the instrument connector.

Each of the two switches 116, 118 has a 10 KΩ shunt resistor 124, 126,respectively, connected across the switch. Accordingly, when theswitches are open, the discharge energy generation circuit is completedby the two shunt resistors and when the switches are closed the circuitis completed by two direct connections.

Both of the two mechanical switches and the MOS transistor switch canassume two impedance states: a high impedance state when open (or off),and a low impedance state when closed (or on). Being all connected inseries, these three switches provide a scheme for sensing theappropriate time to activate the defibrillation discharge circuit and ascheme for verifying the integrity of the switches.

Both the mechanical switches and the MOS switch are normally open; thesmall current flowing in the shunt resistors 98, 124, 126 in thiscondition is low enough that the discharge relay coil 98 is notactivated. In this condition, the voltage produced at the MOS-paddleswitch junction 97 is determined by the bias resistors 98, 100, 102,124, 126 and the supply voltage 82; this is about 2.6 volts for theresistor values shown in FIG. 5. Closure of either paddle switch 116, or118 will remove one of the corresponding resistors 124, or 126 from thebias network by short it out. The resulting sense voltage 114 willtherefor change to about 1.9 volts and be sensed by the microprocessor.The sensing of this condition triggers the microprocessor to indicatethat the switches should be tested for failure.

When an operator presses both paddle switches, the MOS transistor isconnected to ground, and the sense voltage decreases to a low value, ator near ground. Upon detection of this low voltage, the microprocessortriggers the gate 96 of the MOSFET, thereby turning the transistor on.Then with all three of the switches closed, the discharge relayactivates.

When both switches are open, the MOS transistor may be tested in twoways. First, if it has failed in an "on" condition it will short out itsshunt resistor 98, thereby increasing the sense voltage to a high level,between 4.0 and 4.5 volts, which is set by the zener diode 106 and theprotection resistor 102. Second, if the transistor is in the "off"state, it may by turned "on" for a short time, say 2 msec, by themicroprocessor, and the resultant voltage change monitored to check itsintegrity--if functioning the sense voltage would increase. Similarly,if the relay coil is broken, or its connector has a faulty contact, theloss of the bias voltage 82 will change the sense voltage 114 toindicate such a fault.

Table 1 below tabulates the six possible different voltage values at thesense line based on the six possible states of the three switches whenthe electrode resistors have the same shunt resistor values, as in FIG.5.

                  TABLE 1                                                         ______________________________________                                                      Transistor/Relay States:                                                      Trans.   Trans.  Coil                                           Mech. Switch States:                                                                          Off        ShortedOpen                                        ______________________________________                                        Both Open       2.57 V     4.31 V  0.72 V                                     Both Closed     0.18 V     0.55 V  0.18 V                                     One Open        2.04 V     4.30 V  0.58 V                                     ______________________________________                                    

Based on the voltage values of Table 1 and a possible power supply rangeof 8 V-12 V, Table 2 below gives ranges for the sense voltage whichindicate each of the various switch states, as well as electrode andtransistor conditions.

                  TABLE                                                           ______________________________________                                        Switch States:      Voltage Range                                             ______________________________________                                        Trans. off, mech. switches open                                                                   2.3 V-2.9 V                                               Trans. off, one mech. switch open                                                                 1.7 V-2.3 V                                               Trans. off, mech. switches closed                                                                 0.1 V-0.3 V                                               Relay coil unplugged                                                                              1.0 V-1.4 V                                               Transistor shorted  >4.0 V                                                    ______________________________________                                    

Ideally, for a given power supply voltage, the three shunt resistors arechosen such that the voltage range for each of the possible switchconditions is distinct and the various ranges do not overlap. Forexample, the condition of both mechanical switches closed and thetransistor turned off corresponds to a voltage range of 0.1 V-0.3 V,from Table 2, while the condition of one mechanical switch stuck closedand the transistor turned off corresponds to a voltage range of 1.7V-2.3 V. The large differential between these two ranges ensures thatthe microprocessor does not misinterpret the switch conditions.

Further differentiation between each possible switch state could beaccomplished using a different valued shunt resistor for each of thethree resistors. Then eight different switch combinations may bedetected. More generally, any circuit element having a characteristicimpedance or voltage differential may be used in place of the shuntresistors. For example, a zener diode with a characteristic voltage dropmay be shunted across the switches. In addition, circuit elements may beonly temporarily connected in the circuit across the switches while thefunctionality of the switches is tested, rather than permanentlyinstalled in the circuit. In either case, the shunt resistance schemeallows for both precise triggering of the defibrillation dischargecircuit and testing of the circuit's functionality.

Referring now to FIG. 6, the defibrillation discharge circuit 30 isshown in detail. When the discharge energy switch 36 is closed acrossthe circuit, the discharge energy capacitor 34 is discharged to thedefibrillation electrode paddles 18, shown schematically. The capacitorforms an RLC circuit with the pulse shaping inductor 38, the thoracicresistance 130 of the patient to which the paddles 18 are connected, anda parasitic winding resistance 132 corresponding to the inductorwinding. The characteristic values of each of the capacitive, resistive,and inductive components in the circuit determine the energy and shapeof the defibrillation output pulses.

As the capacitor 34 is discharged, current flowing through the inductorwinding generates lines of magnetic flux 134 around the winding turnsand through the winding core. Taking advantage of this, a flux sensecoil 136 is positioned with respect to the inductor such that thecurrent in the inductor, and thus the current in the discharge circuit,can be monitored without direct connection to the circuit. The fluxsense coil 136 comprises a conductor which encircles theinductor-produced lines of magnetic flux 134 at least once; electricalvoltage is thereby developed in the sense coil in correspondence to thelevel of magnetic flux change, and thus, the level of current change inthe inductor.

This flux coupling between the inductor and the sense winding isdetected by circuitry comprising a bias voltage supply 138, set at 5volts, two bias resistors 140, 142, set at 2 KΩ and 500 KΩ,respectively, and a smoothing capacitor 144, set at 0.1 microfarads. Avoltage sense line 146 senses the voltage of the circuitry, whichdepends on the level of flux coupling in the sense coil and the designof the coil; the greater the number of sense coil turns, the higher thecorresponding current and voltage. The voltage sense line is connectedto a monitoring circuit, here a microprocessor, for determination of thedischarge circuit current level, and hence the discharge pulse currentwaveform.

The microprocessor determines the pulse current waveform by sampling thevoltage sense line 146 over the duration of the capacitor discharge toproduce digitized voltage values. Then the voltage values arenumerically integrated over the duration of the discharge and scaled bythe inverse of the inductor value to determine corresponding currentlevel values. These current values, which represent the pulse waveformcurrent, are stored for further processing, such as peak currentdetection, discharge circuit load resistance determination, or pulseenergy determination.

In place of a digital processor such as the microprocessor, hardwarecircuitry may process the sense coil signals to determine the dischargecircuit's current waveform. Voltage values of the sense voltage line 146are here digitized by a hardware analog-to-digital convertor and thenintegrated and scaled by an operational amplifier configured as anintegrator. Peak current detection, load resistance determination, andenergy determination processing may then be accomplished using furtherhardware circuit schemes.

As shown in FIGS. 7A and 7B, the pulse shaping inductor 38 isconstructed of conductive windings wound around an air core 150. Eachwinding layer is isolated by an insulating layer 152 of epoxyimpregnated paper. Exit leads 154 for series connection in the dischargecircuit are located at each end of the winding, which completes 1000turns around the air core.

The sense coil 136 is wrapped around the outer winding of the inductor,and is electrically isolated from the inductor by several layers ofepoxy impregnated paper. Other isolation schemes may be used, but mustwithstand the 7500 V which can develop between the coil and windingbefore breaking down. Sensing exit leads 156 are connected at each endof the coil for connection to the sense circuitry.

Additionally, the sense coil should occupy minimum space to keep theunit small and light. To effectively meet all of the above requirements,the sense coil is formed of a 1/8 inch-wide copper band comprising a0.005 inch-thick copper layer and a 0.0015 inch-thick adhesive layer.This coil configuration is advantageously flat and easily positioned onthe inductor winding. However, any effective conductor which can meetthe discharge circuit's electrical requirements may be used as the coil,in any desired number of turns which generates a desired level of sensecurrent and voltage.

Referring now to FIG. 8, the portable unit 10 includes design featuresfor conveniently discharging the defibrillation discharge circuit totest its functionality. As shown in FIG. 2, the discharge circuit isconnected via either a conventional mating connector or a multifunctionconnector to a pair of defibrillation electrodes. If the conventionalscheme is used, the discharge circuit connects to a pair of hand helddefibrillation paddles 18, shown in FIG. 8. Each paddle supports adefibrillation electrode 160 on its underside for application to thechest of a patient.

When not in use, the defibrillation paddles 18 are stored in a well 162on the portable unit 10. The top side of the paddles are slid under aledge 164 for securing the paddles in place on the unit housing with theelectrodes 160 facing the housing. At the back of the well 162 is aconducting shorting bar 166 which lies on the well bottom and extendslaterally under the ledge 164 to lie under the electrodes 160 when thepaddles are pushed completely into the well. The lateral edges of theshorting bar 166 are flared upward to exert a positive contacting forceon the electrodes when they are in place over the bar.

When the defibrillation discharge circuit is discharged and theelectrode paddles 18 are in place in the well such that the electrodes160 contact the shorting bar 166, a defibrillation pulse is dischargedacross a short circuit, rather than the thoracic resistance of apatient. As explained below, this discharge pulse is monitored toanalyze its characteristics for verifying the acceptability of thepulse. The location of the shorting bar in the paddle well 162 isparticularly advantageous because it prevents contact to the bar by anoperator during a test discharge. As discussed above, the defibrillationcircuit signals may reach quite high levels of voltage and energy, andthus operator coupling to the circuit must be prevented. In its positionin the paddle well, the shorting bar is covered by the paddles andinaccessible to operator contact. However, the shorting bar may belocated at any convenient position on the portable unit, or evendetached from the unit.

Referring again to FIG. 6, during a discharge circuit test into theshorting bar, the thoracic load resistance 130 is replaced by a shortcircuit. As the defibrillation pulse is discharged into the shortcircuit, the flux sense coil 136 is coupled to the discharge inductor 38to produce a sense voltage, and the sense voltage line 146 is monitoredby a microprocessor to analyze the discharging pulse. Alternatively, acurrent transformer, or some other sensing component connected in thecircuit could monitor the current.

In the sense coil scheme described above, once the defibrillationdischarge is initiated, the microprocessor (connected to the sensevoltage line) digitally samples the sense voltage line at 60 microsecondintervals and stores the sampled values. The discharge is ended after 25milliseconds when the discharge capacitor switch is disconnected fromthe inductor discharge circuit. A plot 170 of the stored voltagesamples, as shown in FIG. 9A, illustrates that the voltage initiallyrises to some peak voltage 172, after which it falls to a valley voltage174, and then levels off at a final voltage 176. The peak voltage 172 istypically reached about 6 milliseconds to 8 milliseconds after thedischarge has been initiated, and the final voltage 176 is typicallyreached about 20 milliseconds later.

The microprocessor post-processes the stored voltage data to reconstructthe voltage waveform and convert it to a corresponding current waveform.First each voltage sample is measured to locate the first samples havinga voltage value that is at least about 25% greater than the previousvalues. This indicates the start of the voltage waveform. Then a numberof the previous values, say 6, are remeasured and averaged to determinethe baseline, or quiescent, value of the waveform. This baseline isdefined to be zero volts.

Having determined the waveform baseline, each sample value starting withthe first sample above baseline is numerically integrated and scaled toproduce current values corresponding to the voltage values. As part ofthe integration computation, the baseline value is subtracted from eachintegrated value so that only the waveform value is added in theintegration. At the end of the integration interval, the last currentvalue should be zero. If it is not zero, the last current magnitude ismeasured and proportionately subtracted from all the previous currentvalues to set the baseline value at zero.

A plot of the corresponding current waveform as shown in FIG. 9Billustrates that the current rises to a peak current value 182 and thenfalls off to a valley current value 184, after which it settles to afinal current value 186, which is adjusted to be zero. The current peak182 corresponds to the first zero crossing 173 of the voltage waveformand the current valley 184 corresponds to the second zero crossing 175of the voltage waveform. The magnitude of the current peak and valley isdetermined by the value of the discharge inductor's parasitic resistance(132 in FIG. 6), and thus any changes in the resistance are reflected inthe sampled values. As discussed below, the waveform values are adjustedaccordingly to compensate for any resistance changes.

The current peak value 182 is detected by measuring the current valuesamples located between 0.5 milliseconds and 2.5 milliseconds. Once apeak value is located, it is added to the three current samplespreceding it and the three current samples following it to produce a 7sample peak sum. Similarly, the current valley value 184 is detected bymeasuring the current value samples located between 3.0 milliseconds and6.0 milliseconds. Once a valley value is located, it is added to thethree current samples preceding it and the three current samplesfollowing it to produce a 7 sample valley sum. The peak and valley sumsare then stored for later computations.

A ratio of the peak current sum to the valley current sum is thencomputed. This ratio is used to determine any necessary adjustments inthe peak current to compensate for changes in the discharge inductor'sparasitic resistance. As mentioned above, this resistance may vary; itis particularly dependent on temperature. At 25° C., the resistance ofthe 1000 turn winding is about 13 Ω, but the resistance changes by -0.4%for each °C. drop. Thus, for the range of expected operatingtemperatures, the parasitic resistance ranges from 10 Ω-14 Ω.

As the parasitic resistance varies, the peak current varies accordingly,because the resistance is part of the RLC circuit which generated thepulse current. The valley current also varies according to the parasiticresistance, and is more strongly effected by resistance changes than isthe peak current. With a high load resistance, such as the typical 50 Ωthoracic patient resistance, the current waveform would exhibit novalley current. At a load resistance of about 25 Ω, the circuit would becritically damped, and exhibit a slight current valley. With a shortcircuit load, as in the discharge test described above, the magnitude ofthe valley reaches about 30-35% of the peak current magnitude. If theparasitic resistance decreases due to a temperature decrease, the valleycurrent will increase more than the peak current will increase and willthereby produce a smaller peak to valley current ratio than at a highertemperature. In the limiting case, if the resistance of the RLC circuitdropped to zero, the current peak and valley would be equal and thewaveform would never dampen out.

The current peak is adjusted based on a comparison of the computed peakto valley current ratio and a range of prespecified peak to valleycurrent ratios. At 25° C. and a discharge energy of 100 joules, theratio is expected to be 3.1. If the temperature is below 25° C. thatratio will be lower than 3.1, while the ratio will be higher than 3.1 ifthe temperature is above 25° C.; the change in the parasitic resistancedescribed above effects this ratio change.

If the computed ratio is less than 3.1, then the adjusted peak currentsum I_(peak) ' is computed based on the measured peak current I_(peak)as follows:

    I.sub.peak '=I.sub.peak +7*[13*|R|-40];

if the computed ratio is more than 3.1, an adjusted peak current sumI_(peak) ' is computed based on the measured peak current I_(peak) asfollows:

    I.sub.peak '=I.sub.peak +7*[10*|R|-31],

where R is the peak to valley current ratio.

The peak current sum is not adjusted if the computed ratio is more than4.0. This ratio value indicates that a defibrillation discharge to aresistive load (such as a patient), and not a test discharge to theshorting bar, occurred. As explained above, the load resistance of thedischarge circuit determines the peak and valley pulse currents, andthus the peak to valley current ratio. When a defibrillation pulse isdischarged to a patient, there is little or no current valley, and theratio is above 4.0. Thus no special test indicator must beoperator-activated when a test is to be performed; the microprocessorautomatically detects this via the peak to valley current ratio. Allthat is required is for the energy of the test to be set at 100joules--the current waveform computations are based on this energyvalue. If the ratio is above 4.0, no test results are displayed to theoperator.

For test discharges, the adjusted peak current value is compared to apreset range of current values to determine if the defibrillation pulseis acceptable. If the peak current falls between 65 A and 78 A, themicroprocessor displays a "Test OK" message on the portable unit'sscreen. Conversely, if the peak current does not fall within the range,a "Test Failed" message is displayed for the operator.

Further waveform analysis may be completed. For example, the totalenergy of the pulse may be computed by numerically integrating the pulsecurrent value, squared, multiplied by the inductor parasitic resistanceover the duration of the pulse. Based on this energy value, and theinitial capacitor voltage sample value, the capacitor value may beverified.

Other waveform analysis includes determining the pulse width bydetecting the zero crossings of the current waveform. Additionally, therise and fall times of the waveform may be computed based on the voltagesample values and their corresponding sample times.

As an alternative to the microprocessor, analog and digital hardware maybe implemented to perform the various waveform computations. Forexample, as described above, an operational amplifier configured as anintegrator may be used to complete the various integration calculations.Other hardware circuitry corresponding to the sampling, scaling, andcomparing functions of the microprocessor may similarly be implemented.

Other embodiments are within the following claims. For example, certainof the features relating to the defibrillator could be incorporated in aunit not having pacemaking capability.

What is claimed is:
 1. Apparatus for transcutaneously defibrillating afibrillating heart of a patient, the apparatus comprising:electricalenergy discharge circuitry means for generating defibrillation energypulses discharged at a discharge circuitry output and a pair ofelectrodes; electrical connections from the discharge circuitry outputto said pair of electrodes adapted for application to the patient,discharge control circuitry for initiating a said energy pulse, amanually-operable switch associated with each electrode, said switchbeing normally open and configured to be manually closed by an operator,the two switches connected in series with said discharge controlcircuitry, said control circuitry being configured so that when theswitches are both closed a defibrillation energy pulse is initiated,first and second circuit elements each having a characteristic impedanceand connected in parallel with a corresponding one of the switches sothat each switch shorts out a corresponding circuit element when thatswitch is closed, means for measuring an indication of the seriesimpedance of the discharge control circuitry to thereby determine theopen or closed state of the switches by comparing a measured seriesimpedance value indication with a series impedance value indicationcalculated for a given open or closed state of each of the switches, andsaid discharge control circuitry including means for detecting theclosed state of either of said switches at a time when said switchesshould be open and for indicating such a failure detection to the userof the apparatus, and further for preventing a discharge of a saiddefibrillation pulse on detection of such a failure.
 2. The apparatus ofclaim 1 wherein said discharge control circuitry comprises an electricalcomponent switch that is normally inactivated and that can beelectronically activated, the electrical switch connected in series withthe discharge control circuitry such that when the two manually-operableswitches are simultaneously closed and the electrical switch isactivated the discharge control circuitry initiates a said energy pulse,and further comprisinga third circuit element having a characteristicimpedance and connected in parallel with the electrical component switchso that the component switch shorts out the third circuit element whenthe component switch is activated, said discharge control circuitryincluding means for detecting the activated state of the electricalswitch at a time when said electrical switch should be inactivated andfor indicating such a failure detection to the user of the apparatus,and further for preventing a discharge of a said defibrillation pulse ondetection of such a failure.
 3. The apparatus of claim 2 wherein theindication of the series impedance of the discharge control circuitry isdetermined based on a measured voltage drop across the switches measuredby the measuring means.
 4. The apparatus of claim 3 wherein each of thefirst, second, and third circuit elements comprises a resistor.
 5. Theapparatus of claim 4 wherein the first resistor and the second resistoreach have the same resistance value, and wherein the measuring meansdetermines the open or closed state of the series combination of the twomanually-operable switches.
 6. The apparatus of claim 4 wherein themeasuring means comprises a voltage sensor which measures the seriesvoltage drop across the three switches.
 7. The apparatus of claim 1wherein each electrode and its associated manually-operable switch islocated on a hand held paddle.
 8. A method for detecting the open orclosed state of a pair of manually-operated switches associated with apair of defibrillation pulse electrodes for application ofdefibrillation energy pulses to a fibrillating heart of a patient, thedefibrillation energy pulses being generated by electrical energydischarge circuitry having an output that is electrically connected tothe pair of electrodes, the switches being normally open and beingconfigured to be manually closed by an operator, the two switchesconnected in series with discharge control circuitry for initiating asaid defibrillation energy pulse, said control circuitry beingconfigured so that when the switches are both closed a defibrillationenergy pulse is initiated, each of the switches being connected inparallel with a corresponding one of first and second circuit elements,each having a characteristic impedance, so that each switch shorts out acorresponding circuit element when that switch is closed, comprising thesteps of:measuring an indication of the series impedance of thedischarge control circuitry to thereby determine the open or closedstate of the switches by comparing a measured series impedance valueindication with a series impedance value indication calculated for agiven open or closed state of each of the switches, and detecting theclosed state of either of the switches at a time when the switchesshould be open and indicating such a failure detection to the user ofthe apparatus, and preventing a discharge of a said defibrillation pulseon detection of such a failure.
 9. The method of claim 8 wherein saiddischarge control circuitry comprises an electrical component switchthat is normally inactivated and that can be electronically activated,the electrical switch connected in series with the discharge controlcircuitry such that when the two manually-operable switches aresimultaneously closed and the electrical switch is activated thedischarge control circuitry initiates a said energy pulse, a thirdcircuit element being connected in parallel with the electricalcomponent switch and having a characteristic impedance, so that thecomponent switch shorts out the third circuit element when the componentswitch is activated, further comprising the step ofdetecting theactivated state of the electrical switch at a time when said electricalswitch should be inactivated and for indicating such a failure detectionto the user of the apparatus, preventing a discharge of a saiddefibrillation pulse on detection of such a failure.
 10. The method ofclaim 9 wherein the step of measuring an indication of the seriesimpedance of the discharge control circuitry comprises measuring avoltage drop across the switches.
 11. The method of claim 10 whereineach of the first, second, and third circuit elements comprises aresistor.
 12. The method of claim 11 wherein the first resistor and thesecond resistor each have the same resistance value, and wherein themeasuring step determines the open or closed state of the seriescombination of the two manually-operable switches.
 13. The method ofclaim 11 wherein the measuring step comprises a measuring the seriesvoltage drop across the three switches.
 14. The apparatus of claim 8wherein each electrode and its associated manually-operable switch islocated on a hand held paddle.